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This is White's first tesuji in the upper left corner.
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Black must answer from above; otherwise his entire group will die.
: (A 566ca)
Black might consider connecting at , instead, aiming at an ishi-no-shita shape in the corner, but this move will result in a disaster for him.
:
This placement is White's second tesuji.
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Black has to intercept, where after White sacrifices another stone in order to spoil Black's eye shape.
:
White's throw-in is the final blow. :
Black is unable to capture at , otherwise White's cut / atari at will leave him with only one eye.
Therefore, Black captures two White stones in the corner, saving at least some of his stones in the upper left.
White gained less than 25 points in this corner, due to Black's initial mistake.
:
This move starts the remaining endgame.
Black gained over ten points in the lower right corner and at the buttom, so the net gain is over ten points for White.